1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for examining patients. The present invention is more specifically directed to a nuclear magnetic resonance tomography apparatus as well as to a method for operation thereof wherein the MRT data are acquired by a technique known as Partially Parallel Acquisition (PPA).
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized as an imaging method in medicine and biophysics for more than 15 years. In this examination modality, the subject is exposed to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject align, these having been previously irregularly oriented. Radiofrequency waves then excite these xe2x80x9corderedxe2x80x9d nuclear spins to a specific oscillation. This oscillation generates the actual measured signal in MRT, this being picked up with suitable reception coils. The measurement subject can be spatially encoded in all three spatial directions by utilizing inhomogeneous magnetic fields generated by gradient coils. MRT allows a free selection of the slice to be imaged, so that tomograms of the human body can be registered in all directions. In medical diagnostics, MRT is particularly distinguished as a tomographic imaging method that is non-invasive with a versatile contrast capability. Due to the excellent presentation of soft tissue, MRT has developed into a modality that is often superior to X-ray computed tomography (CT). MRT is currently based on the application of spin echo frequencies and gradient echo frequencies that enable an excellent image quality with measurement times on the order of magnitude of minutes.
The ongoing technical improvement of the components of MRT systems and the introduction of fast imaging sequences have made an increasing number of areas of application in medicine accessible to MRT. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples. Despite the technical advances in designing MRT systems, the exposure time needed for an MRT image remains the limiting factor for many applications of MRT in medical diagnostics. Limits are placed on a further enhancement of the performance of MRT systems with respect to the exposure time from a technical point of view (feasibility) and for reasons of patient protection (stimulation and tissue heating). Numerous efforts therefore have been made in recent years to further shorten the image measurement time by means of new approaches.
One approach for shortening the acquisition time is to reduce the quantity of image data to be acquired. In order to obtain a complete image from such a reduced dataset, however, the missing data either must be reconstructed with suitable algorithms or the faulty image must be corrected from the reduced data.
The registration of the data in MRT occurs in k-space (frequency domain). The MRT image in the image domain is linked to the MRT data in k-space by means of Fourier transformation. The location coding of the subject, who defines the k-space, occurs by means of gradients in all three spatial directions. A distinction is made between the slice selection (determines an exposure slice in the subject, usually the z-axis), the frequency coding (determines a direction in the slice, usually the x-axis) and the phase coding (determines the second dimension within the slice, usually the y-axis).
A k-space that is represented in polar coordinates and scanned projection-by-projection is assumed below. The data of an individual k-space row are frequency-coded with a gradient upon readout. An acquisition method for projection reconstructions employs a gradient that does not scan line-by-line in the Cartesian format, but rotates around the specimen. A projection through the entire specimen from a specific direction thus is obtained in every measuring step, as is a typical dataset for the projection reconstruction in k-space, as shown in FIG. 5. The totality of points corresponding to the registered data in k-space is referred to below as a projection dataset. As mentioned above, the entire projection dataset must be projected onto a Cartesian grid in order to transform the k-space data into an MRT image by means of Fourier transformation. A conversion of the registered projection dataset into such a grid raster of a Cartesian coordinate system in shown in FIG. 5. The grid constant of the grid raster is defined by the projection angle as well as by the size of k-space. Using an approximation method (interpolation method), the values of the grid intersections are interpolated on the basis of the most proximate points in the projection dataset. Each projection in k-space has the spacing xcex94xcex1 that is generated by a phase-coding step of the rotating gradient. Since the phase-coding takes a great deal of time compared to the other location codings, most methods for shortening the image measurement time are based on a reduction of the number of time-consuming phase-coding steps. All methods of employing a known technique referred to as partially parallel acquisition (PPA) are essentially based on the above approach.
The basic idea in PPA imaging is that the k-space data are not acquired by an individual coil but by component coils, for example arranged annularly around the subject. Each of the spatially independent coil arrays carries certain spatial information that is utilized in order to achieve a complete location encoding via a combination of the simultaneously acquired coil data. This means that a number of xe2x80x9comittedxe2x80x9d projections can be defined in k-space from a single, registered k-space projection.
The PPA methods thus employ spatial information that is contained in the components of a coil arrangement in order to partially replace the time-consuming forwarding of the rotating gradient. As a result, the image measurement time is reduced, corresponding to the ratio of the number of projections of the reduced dataset to the number of rows of the conventional (i.e. complete) dataset. In a typical PPA acquisition, only a fraction (xc2xd, ⅓, xc2xc, etc.) of the projections is acquired compared to the conventional acquisition. A specific reconstruction technique is then applied to the projection data in order to reconstruct the missing projections, and thus to obtain the full field of view (FOV) image in a fraction of the time. The FOV is defined by the size of k-space under observation according to the factor 2xcfx80k.
Whereas various PPA methods (SMASH, SENSE) have been successfully employed in many areas of MRT. A disadvantage of these methods is that the complex coil sensitivities of every individual component coil must be exactly known. Conventionally, the coil profiles have been acquired with additional measurement steps. These additional measurement steps can be composed of an additional sequence (for example of a 3D sequence) or can be integrated into the actual measurement sequence. A disadvantage of the previous methods thus is generally in a lengthening of the measurement duration.
It is an object of the present invention to shorten the overall measurement time of a parallel acquisition method for projection reconstructions. In particular, the acquisition time for the coil sensitivities should thereby be shortened.
This object is inventively achieved in a method for magnetic resonance imaging on the basis of a partial parallel acquisition (PPA) for projection reconstructions by exciting nuclear spins and measuring radiofrequency signals arising from the excited spins in the form of projection datasets, the method including the steps of under-scanning k-space with component coils and forming a complete projection dataset by means of progressive rotation of the readout gradient through angles of fixed steps, producing a Cartesian resultant dataset by transforming the projection dataset onto grid points of a first Cartesian grid with a first grid constant, producing a Cartesian reference dataset by transforming a central region of the projection dataset onto a second Cartesian grid with a grid constant smaller than that of the first grid, calculating the sensitivities of the component coils on the basis of the reference dataset, and reconstructing an output image on the basis of a dataset obtained by linking operation of the calculated coil sensitivities of the component coils with the resultant dataset.
The grid constant of the first Cartesian grid is defined by the step angle and the size of k-space.
Upon doubling of the step angle, in particular, the size of the central region is defined by halving the diameter of the complete projection dataset.
Upon doubling of the step angle, the grid constant of the second Cartesian grid is likewise defined half as large as the grid constant of the first Cartesian grid.
The reconstruction of the output image after the linking of the coil sensitivities of the component coils calculated from the reference dataset with the resultant dataset ensues by means of Fourier transformation.
The component coils can be arranged in both k-space directions.
A further advantage is obtained with an arrangement of the component coils on a ring around the slice to be measured and in the plane thereof.
A magnetic resonance tomography apparatus for the implementation of this method is also described.
The invention also includes a computer software product that implements an above method when it is loaded in a computer device of the magnetic resonance tomography apparatus.